WO2003054046A1 - Method for producing laminated materials based on wood veneer - Google Patents

Abstract

The invention relates to a method for producing multi-layer laminated materials based on wood veneer, by pressing veneered wood which is glued by means of bonding agents. According to the invention, the cold-bonding strength of the laminates is increased by means of specific bonding agents.

Description

Manufacturing process for wood veneer based laminates

The invention relates to a process for the production of multilayer laminates based on wood veneer by pressing veneer wood glued with binder, in which the cold adhesive strength of the laminates is increased by means of special binders.

Pressed materials as finishing products of veneers from wood types of all growth regions (e.g. pine, okumee, poplar) and binders are processed by pressing at different temperatures to wood laminates such as 'Laminated Veneer Lumber' (LVL), plywood, plywood, star wood or blockboard. They play an important role as high-quality materials for the furniture, construction and packaging sectors.

Important binders in the production of these laminates are amino resins (urea-formaldehyde resin (UF), melamine-urea-formaldehyde resin (MUF)) or phenolic resins (PF). Disadvantages of these condensation resins are the sometimes long pressing times (MUF, PF), dark adhesive joints (PF), high pressing temperatures (MUF, PF) and inadequate water resistance (UF).

With PUR plywood binders, however, the disadvantages mentioned above can be avoided. Disadvantages of these PUR binders, however, are again a very poor cold adhesive strength of the glued laminates and the penetration of the binder onto the surface of the top layer of the plywood ("penetration of the binder"). Thus, pMDI moves completely into the wood grain without an adequate adhesive joint on the To form veneer surface.

Various proposals have been made to solve this problem. According to EP-A 352 558, the cold adhesive strength is determined by using polyisocyanates, compounds with at least two isocyanate-reactive water atoms and alkylene carbonates improved. EP-A 93 357 describes prepolymers based on urethane-modified diphenylmethane diisocyanates as binders. WO 99/19141 discloses binders made of diphenylmethane diisocyanates and isocyanate-reactive polymers for plywood production by veneer pressing. However, the initial strength when cold leaves a lot to be desired.

It has now been found that the cold adhesive strength in the processing of the glued veneers can be considerably improved if pMDI prepolymers are used as binders which are urethane-modified by reaction with EO-rich hydroxy-functional polyethers.

The present invention relates to a process for the production of pressed materials on the basis of multi-layer wood veneers and prepolymers as binders, in which prepolymers containing urethane groups with an NCO content of 20 to 31% by weight are used as binders, which are obtained by reacting polyisocyanates the diphenyl membrane diisocyanate series with hydroxy-functional polyethers with an EO content of more than 60% by weight, based on the total amount of alkylene oxides used to prepare the polyethers, are obtained.

Polyisocyanates of the diphenylmethane diisocyanate series are used to prepare the prepolymers, e.g. Polyphenyl-polymethylene-polyisocyanates, such as those produced by aniline-formaldehyde condensation and subsequent phosgenation ("crude MDI"). Higher-core isocyanates of the diphenylmethane diisocyanate series (pMDI types) are preferably used as the polyisocyanate component. These particularly preferably have a monomeric diphenylmethane diisocyanate content of less than 55% by weight.

The prepolymers are obtained by reacting the polyisocyanates with hydroxy-functional polyethers with an EO content of more than 60% by weight, preferably more than 70% by weight, based on the total amount of alkylene oxides used to prepare the polyethers, is obtained. The hydroxy-functional polyethers generally have 1 to 8, preferably 2 to 6, OH groups. They preferably have number average molecular weights of 400 to 10,000 g / mol, particularly preferably 1,000 to 8,000 g / mol. Poly (oxypropylene-polyoxyethylene) polyols are preferably used.

The hydroxy-functional polyethers can be prepared by known processes, for example by anionic polymerization of alkylene oxides in the presence of starter compounds containing active hydrogen atoms. Suitable alkylene oxides contain 2 to 4 carbon atoms in the alkylene radical. Examples include, in addition to ethylene oxide, tetrahydrofuran, 1,2-propylene oxide, 1,2- or 2,3-butylene oxide; ethylene oxide (EO) and 1,2-propylene oxide (PO) are preferably used. The alkylene oxides can be used alternately in succession or as mixtures.

As starter compounds having active hydrogen atoms, compounds having (number average) molecular weights of 18 to 2,000 g / mol and 1 to 8 hydroxyl groups are preferably used. Examples are ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,4-butanediol, hexamethylene glycol, bisphenol A, trimethylolpropane, glycerin, pentaerythritol, sorbitol, cane sugar, degraded starch or. Water. It is also possible to use oligomeric alkoxylation products of the abovementioned low molecular weight starters with (number average) molecular weights of 200 to 2,000 g / mol as starter compounds.

The reaction of polyisocyanates and hydroxy-functional polyethers is usually carried out at temperatures from 20 to 120 ° C. Polyisocyanates and hydroxy-functional polyethers are used in proportions such that the prepolymer formed has an NCO content of 20 to 31% by weight, preferably 22 to 28% by weight. The prepolymers preferably have an equivalent weight of

250 to 5,000 g / mol. Suitable raw materials that can be bound with the binder according to the invention are wood veneers, for example from wood from the boreal, subtropical or tropical regions. In addition, plastics can also be used. These plastics can be in the form of thin laminates, sheets or foils.

Papers, cellulose, fabric webs, nonwovens, as well as barrier or decier diapers can also be used to build up the laminates.

The binder is applied to the raw material in an amount of 40 to 250 g / m ² , preferably 60 to 180 g / m ^{2, of} wood surface and - generally under the action of pressure and heat (for example temperature 70 to 250 ° C. and more specifically Pressure 1 to 50 bar) - pressed into sheets or moldings. The procedure is often to use an odd number of veneers to make the plywood. Thinner veneers are often used for the outer layers than for the inner area of the plywood. Only the even-numbered layers are exposed to the binder on both sides. By locking together the alternately glued and unglued layers and then hot pressing them together, the veneer layers are connected to each other to form a plywood. At LVL, on the other hand, all veneers are laid parallel in the production direction and hot pressed after the continuous pretreatment.

The gluing is usually carried out with a 4-roll gluing machine. Just like condensation resins, PUR binders according to the invention can also be applied using this application technique and other techniques such as coating, spraying (airless, two-component) or knife coating.

In one embodiment of the invention, the veneers are applied with water in the form of a spray after the gluing. The amount of water can be up to 150 g per m ^{2 of} laminate surface, preferably it is 10 g / m ² to 80 g / m ² . To avoid premature curing of the binder (extension of the 'pot life') and to precisely adjust the viscosity of the PUR binder zeolites can be added to the binder. The amount added can be 0.3 to 25% by weight, based on the total amount of the binder, but is preferably 1 to 5% by weight.

Subsequent pre-pressing brings the glued and unglued veneer layers together to form a bond to enable the plywood that is still to be hot-pressed to be easily transported to the hot press. The distance-controlled cold pre-press is closed to approx. 0.1 to 0.4 mm above the calculated sum of all veneer layers and therefore does not lead to a compression, but only a composite of the veneer system. The network must be one for transport to the

Hot press have sufficient cold adhesive strength.

During the subsequent hot pressing at 70 ° C - 200 ° C, the veneer layers are bonded together with the binder that hardens at elevated temperature. Since the PUR binders used do not contain water, the water balance of the

Veneer layers must be set exactly according to individual needs by the drying process or by adding water spray. Productivity increases can be achieved by shortening the pressing process thanks to an optimal water balance.

Examples

A. Prepolymers

To prepare the prepolymers, the respective components were mixed and then heat-treated at 95 ° C. for two hours with constant stirring.

Prepolymer 1

Prepolymer of 840 g of polymeric MDI (, pMDI ') having an NCO content of about 31.5 wt .-% (Desmodur ^® A20 1520, Bayer AG) and 210 g of a Polyetherpoiyols an OH number of 36 glycerol-initiated with 85% primary OH groups,

PO / EO ratio 28% by weight / 72% by weight. The prepolymer has an NCO content of 24.5% by weight and a viscosity of 1400 mPa-s (25 ° C).

Prepolymer 1 A 50 g of zeolite L (UOP-L powder, UOP GmbH, D-51368

Leverkusen) dispersed.

Prepolymer 1 B

100 g of zeolite L (UOP-L powder, UOP GmbH, D-51368 Leverkusen) were dispersed in 900 g of prepolymer 1.

Prepolymer 1 C

150 g of zeolite L (UOP-L powder, UOP GmbH, D-51368 Leverkusen) were dispersed in 850 g of prepolymer 1.

Prepolymer 2

Prepolymer of 840 g Desmodur ^® A20 1520 and 160 g of a Polyetherpoiyols an OH number of 100 initiated with sorbitol with 90% primary OH groups, PO / EO ratio 18 wt .-% / 82 wt .-%. The prepolymer has an NCO content of 25.5% by weight and a viscosity of 2500 mPa-s (25 ° C). Prepolymer 3

Prepolymer of 775 g Desmodur ^® A20 1520 and 225 g of a Polyetherpoiyols an OH number of 25 glycol initiated Butyldiethylen- with 90% primary OH groups, PO / EO ratio 15 wt .-% / wt .-% 85. The prepolymer has an NCO content of 23.4 wt .-% and a viscosity of 620 mPa-s (25 ° C).

Prepolymer 4

Prepolymer made from 266 g Desmodur ^® 1520 A20 and 60. g of a polyethylene oxide with OH number 73 started with ethylene glycol. The prepolymer has an NCO content of 24.3% by weight and a viscosity of 1300 mPa-s (25 ° C.).

Prepolymer 5

Prepolymer of 840 g Desmodur ^® 1520 A20 and 160 g of a polyether polyol started with propylene glycol with an OH number of 185 and more than 90% primary OH groups, PO / EO ratio 3% by weight / 97% by weight. The prepolymer has one

NCO content of 24.1 wt .-% and a viscosity of 3100 mPa-s (25 ° C).

Prepolymer 6 (comparison)

Prepolymer made from 1200 g Desmodur ^® 1520 A20 and 300 g of a polyether polyol of OH number 28 started with trimethylol propane with 85% primary OH

Groups, PO / EO ratio 85 wt% / 15 wt%. The prepolymer has an NCO content of 24.4 wt .-% and a viscosity of 1240 mPa-s (25 ° C).

Prepolymer 7 (comparison) Prepolymer made from 1200 g Desmodur ^® 1520 A20 and 300 g of a polyether polyol of OH number 28 started with propylene glycol with 85% primary OH groups, PO / EO ratio 50% by weight / 50% by weight , The prepolymer has an NCO content of 23.8% by weight and a viscosity of 1200 mPa-s (25 ° C). B. gluing and cold pressing of veneers

Veneers made of Okumee wood with a density of 0.4-0.5 g / m ³ and a moisture content of approx. 5-7% by weight of dry (absolutely dry wood) were used. Veneers with a thickness of 2.3 mm were used for the middle layer (MS) and veneers with a thickness of 1.2 mm for the top layer (DS). 7-layer laminates from 5 MS

Layers and 2 DS layers as veneer finish.

Veneers measuring 450 mm x 450 mm were loaded with 40 g prepolymer (20 g each on the top and bottom, corresponding to 100 g / m ² ) using a 4-roll gluing machine. In some cases, zeolite L powder was added to the prepolymer to improve processability. The basis weight of approx. 100 g / m ² was maintained in these cases. After the prepolymer had been applied, 8 g of water (corresponding to 20 g / m ² ) were sprayed onto each veneer.

The laminate was cold-pressed at room temperature at 5 to 10 bar

Press pressure. The laminate was compressed to a height of approximately 15.3 mm. The time from the beginning of the gluing to the start of the compression was about 10 minutes, the pressing time was 8 minutes.

C. Determination of the cold adhesive strength

The cold adhesive strength required for transport was checked by lifting the cold pre-pressed laminate, by lifting the top veneer on one side with a knife by approx. 5 cm after removing the additional sheets. Depending on the characteristics of the cold adhesive properties, the bond held together or fell apart immediately or delayed in its individual veneers. The characterization was carried out as follows:

D. Hot pressing of the glued veneers

The hot-pressing process permanently bonded the glued and cold pre-pressed veneers to one another. The energy supplied by the press leads to the crosslinking of the PUR binder. This creates a permanent and hydrolysis-stable PUR binder joint that mechanically connects the veneers on both sides of the adhesive joint.

E. Determination of binder penetration and technical properties

After removing the plywood from the hot press, the quality of the bond was checked visually for surface bonding (blistering, peeling of the veneers). In addition, the penetration behavior ("penetration" of the binder on the plywood surface was assessed. The penetration behavior was divided into the following three classes:

Die Verklebungsquahtät wurde abschließend im Scherversuch festgestellt. Die Probenvorbereitung wurde entsprechend EN 314-1 Teil 5.1.3 durchgeführt. Die Proben wurden entsprechend EN 314-2 - Klasse 3: Außenbereich geprüft. Zum Vergleich wurde ein Nullwert von einem unmodifizierten PUR-Bindemittel ebenfalls ermittelt.

The bonding quality was finally determined in the shear test. The sample preparation was carried out according to EN 314-1 part 5.1.3. The samples were tested in accordance with EN 314-2 - Class 3: Outside. For comparison, a zero value of an unmodified PUR binder was also determined.

The results are summarized in the following table:

n.b. - nicht bestimmt

nb - not determined

The laminates produced with the binders according to the invention already have high adhesive strengths after being pressed at room temperature. No penetration of the binder can be observed in the cover layers.

In addition, some of the plywood laminates were also pretreated and tested in accordance with EN 314-1 Part 5.1.4 in order to be suitable for outdoor use

Confirm EN314-2. The tests showed only a slight influence of the pretreatment on the strength values of the plywood produced by the method according to the invention. After sample preparation by boiling for 72 hours, the strength was only reduced by approx. 15% compared to sample preparation by changing cooking. These results suggest a good one

The samples are resistant to hydrolysis.

Claims

claims 1. Process for the production of laminates based on wood veneer, in which veneers glued with binder, optionally in combination with papers, cellulose or fabric webs, are pressed, whereby as Prepolymers containing binders of urethane groups with an NCO content of 20 to 31% by weight are used, which are obtained by reacting polyisocyanates of the diphenylmethane diisocyanate series with hydroxy-functional polyethers with an EO content of more than 60% by weight, based on the total amount alkylene oxides used to prepare the polyethers are available. 2. The method according to claim 1, in which is used as the polyisocyanate polymeric diphenylmethane diisocyanate (pMDI) with a content of monomeric diphenylmethane diisocyanate of less than 55% by weight. 3. The method according to claim 1, in which water is sprayed onto the veneer glued with binder. 4. The method of claim 1, wherein the binder contains zeolite. 5. Use of prepolymers containing urethane groups with an NCO content of 20 to 31% by weight, which are obtained by reacting polyisocyanates of the diphenylmethane diisocyanate series with hydroxy-functional polyethers with an EO content of more than 60% by weight, based on the Total amount of alkylene oxides used to produce the polyethers are available in the production of wood veneer-based laminates.